The present invention relates to the preparation of bisphenol of improved purity. Bisphenols are used as a starting material for the preparation of various materials, notably epoxy resins and polycarbonates. Bisphenols are usually prepared by the condensation reaction of an aldehyde or a ketone and a stoichiometric excess of a phenol in a reaction mixture in the presence of a catalyst. The product mixture is usually passed to a crystallization zone wherein a crystalline bisphenol product and a mother liquor recycle stream is produced. The mother liquor contains a major amount of a phenol and is usually recycled to the reaction mixture. However, this condensation reaction using any known catalyst always produces a number of by-products. The product mixture contains, in addition to the desired bisphenol, excess phenol, catalyst, unreacted aldehyde or ketone, water, a variety of by-products, such as 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (hereafter referred to as o,p'-bisphenol isomer), 2,2,4-trimethyl-4-(4-hydroxyphenyl)chroman, trisphenol, polyphenol and unfavorably colored substances. A portion of these by-products in the product mixture originates from the recycled mother liquor. The repeated recycling of the mother liquor typically results in a build-up of by-products in the product mixture. These by-products have an unfavorable influence on the quality of the resins prepared from the bisphenols. It is well known that the production of polycarbonates requires a bisphenol of very high purity.
Accordingly, many suggestions have been made to increase the purity of bisphenols, such as bisphenol A (2,2'-bis(4-hydroxyphenyl)-propane), either by separating the by-products from the bisphenol in the product mixture or by purifying the mother liquor to be recycled in order to decrease the concentration of the by-products in the product mixture.
U.S. Pt. No. 4,354,046 relates to process of purifying bisphenol A wherein unreacted phenol, acetone and water is removed from the crude reaction mixture and the remaining crude bisphenol A is mixed with water and toluene. This mixture is heated to form a single liquid phase. The liquid phase is then cooled whereby bisphenol A crystallizes. The remaining liquor is distilled to remove water and toluene. Phenol is added to the remainder. This mixture contains large amounts of the o,p'-isomer of bisphenol A and other by-products. The mixture is passed through a bed of cation exchange resin in acid form to convert most of the by-products to the desired bisphenol A. The effluent from the cation exchange bed can be recycled to the bisphenol reactor.
U.S. Pat. No. 4,107,218 relates to a process of reducing the content of color bodies in the bisphenol A recycle stream. After the separation of the product mixture obtained in the bisphenol A process into a bisphenol A/phenol adduct and a mother liquor recycle stream, the recycle stream is contacted with an acidic cation exchange resin to reduce the content of color bodies contained in the mother liquor stream. The acidic cation exchange resin is periodically reactivated by washing with a phenol/water mixture.
U.S. Pat. No. 4,766,254 teaches that product losses and color formation are sometimes encountered when bisphenol A is produced from phenol and acetone in the presence of a cation exchange resin (which is used as a catalyst). It is taught that the product stream from the reaction is frequently separated into a solid adduct of bisphenol A and phenol and a liquid stream which contains crude bisphenol A and various impurities from which further bisphenol A is recovered by distillation. The U.S. patent further teaches that sometimes substantial product losses occur during this distillation step and that the mother liquor is frequently contaminated with darkly colored materials which are difficult or impossible to remove during bisphenol A purification. In order to suppress such losses and to decolorize the liquid stream which contains crude bisphenol A, the U.S. patent suggests to remove acid impurities from the crude liquid stream by contact with a basic ion exchange resin before further bisphenol A is recovered from the resin-treated crude liquid stream by distillation.
U.S. Pat. No. 4,191,843 teaches a process of producing bisphenols by reacting at least two moles of phenol with acetone in a reaction zone in the presence of an acid ion exchanger, such as a sulfonated ion-exchange resin. The reaction zone effluent is contacted with an acid ion exchanger in metal salt form and/or a weak base ion exchanger. The U.S. patent teaches that the reaction zone effluent is preferably first contacted with an strong-acid ion exchanger before it is contacted with an acid ion exchanger in metal salt form and/or a weak base ion exchanger. After the reaction zone effluent has been contacted with the acid ion exchanger in metal salt form and/or the weak base ion exchanger, water and acetone are removed from the reaction zone effluent and the bisphenol is recovered from the residue of the separation.
German Offenlegungsschrift DE-A-4014992 relates to the production of bisphenols by reaction of a phenol with an aldehyde or ketone. Crude phenol is utilized, which originates from the working up of carbonaceous products, such as brown coal or waste from the hydration of coal or from the working up of waste waters of crackers. Such crude phenol is first activated with a cation exchange resin in acidic form before it is reacted with an aldehyde or ketone in the presence of a sulfonated cation exchanger in acidic form. The resulting reaction mixture is treated with an anion exchanger of weak or average basicity.
Although the purity of the produced bisphenols is considerably improved by the taught processes, their purity is sometimes still not sufficient, especially if the bisphenols are intended to be used for producing polycarbonates. Accordingly, it is still desirable to provide an improved process by which bisphenols can be produced at high purity.